1. Field of the Invention
This invention relates to an electrostatic latent image developing toner and a method for the production thereof. More particularly, it relates to a toner to be used in the development of electrostatic latent images in electrophotography, electrostatic recording, and electrostatic printing and to a method for the production of the toner.
2. Description of the Prior Art
The development of electrostatic latent images in electrophotography, electrostatic recording, and electrostatic printing is effected by causing a triboelectrified toner to be electrostatically deposited on an electrostatic latent image formed on a sensitive material thereby converting the latent image into a visible image.
As means of charging the toner to be used in the development of the electrostatic latent image, the two-component developing method is known to attain the impartation of charge by mixing and stirring the toner with a substance generally called a carrier and the one-component developing method is known to effect the impartation of charge by exposing the toner to contact with a developing sleeve or a toner regulating blade. No matter whichever of these methods may be used, if the charge is not imparted uniformly, problems arise during the course of development and transfer of an image.
Heretofore, the dry toner has been generally produced by a method which comprises mixing a pigment such as carbon black with a thermoplastic resin, melting and kneading the mixture thereby forming a uniform dispersion, and thereafter comminuting the dispersion with a suitable finely dividing apparatus into a powder possessing a particle diameter required of a toner. The individual particles of the toner produced by this comminution method have no fixed shape. This fact tends to cause agglomeration of toner particles, possibly functions as an adverse factor on the stability of toner during storage, the dispensing property of toner during supply, and the clarity and sharpness of a developed image, and brings about a serious problematic effect on the quality of the image to be actually obtained, particularly in terms of resolving power, clarity and sharpness, and fogging.
In recent years, the electrostaic latent image developing toner has been urged to fulfil the requirement that it should warrant production of an image of high fineness of delineation for the sake of repeatability of lines and the requirement that it should be capable of producing an image of high quality in terms of mesh-pattern reproducibility, halftone reproducibility, tonality, and resolving power. To meet these requirements, the toner particles are desired to be amply decreased in diameter. This decrease in diameter of the toner particles, however, goes on the other hand to impair the powder properties such as flowability which are to be displayed by the toner itself or the mixture of the toner with a carrier in the case of two-component developing method. When this decrease of particle diameter is tried on the toner which is obtained by the comminution method described above and, threfore, composed of particles devoid of a fixed shape and wide in diameter distribution, the produced toner suffers from extreme impairment of flowability. Even if the toner is made to incorporate therein a large amount of a suitable after treating agent for enhancing flowability, the incorporation of this agent entails such secondary effects as defective electrification and serious aggravation of toner-scattering.
In contrast to the toner which is produced by the comminution method described above, the toner which is produced by the so-called suspension polymerization method, i.e. by polymerizing a polymer composition composed of a polymerizable monomer, a polymerization initiator, and a coloring agent as suspended in a non-solvent type dispersion medium, as disclosed in Japanese Patent Publication SHO No. 36(1961)-10,231, Japanese Patent Publication SHO No. 43(1968)-10,799, and Japanese Patent Publication SHO No. 51(1976)-14,895, for example, has been also known to the art. This suspension polymerization method is advantageous from the productional point of view because it has no use for any step of comminution. The toner obtained by this method is composed of spherical particles and is generally said to exhibit highly desirable flowability. Since the toner obtained by the suspension polymerization method comprises spherical particles of smooth surface and, therefore, has a smaller surface area than the toner obtained by the comminution method and, consequently, composed of particles devoid of a fixed shape, it is deficient in the toner charging property, a factor dependent on the surface area of toner particles. As the result, poor electrification-build-up, insufficiency of the absolute charge amount of toner and broadening of charge distribution are produced, thereby entailing such adverse phenomena as drifting of toner particles and fogging of the produced image. Further, owing to the sphericity of the toner particles, the toner particles exhibit high adhesiveness to the sensitive material and adhere fast to the sensitive material and also exhibit ready rollability as compared with the toner particles lacking a fixed shape and defy effective removal from the sensitive material during the course of the cleaning treatment.
For the purpose of producing a toner endowed with a composite construction and consequently enabled to discharge separate functions, Japanese Patent Publication SHO No. 59(1984)-38,583 discloses a toner which has a coating layer formed of minute particles by emulsion polymerization and deposited wet on the surface of core particles and Japanese Patent Laid-Open SHO No. 62(1987)-226,162 discloses a toner which is produced by depositing minute resin particles wet on the surface of colored thermoplastic resin cores and subsequently heat-treating the resultant composite cores, for example. These toners invariably embody an idea of harnessing the fact that the electrical properties of a toner depend mainly on the surface portion of toner particles, specifically by depositing minute resin particles on the surface of core particles containing a coloring agent, a magnetic substance, etc. thereby enabling the deposited minute resin particles to improve the surface properties of the core particles and allowing the core particles to acquire a roughened surface, an increased surface area, an increased friction coefficient, and improved charging property. The minute resin particles deposited wet as described above, however, are separated easily from the core particles and, therefore, do not bring about the improvement of surface properties fully as claimed. Further, in the layer of minute resin particles deposited wet as described above, the minute resin particles are deposited as retaining their original particulate form intact on the surface of the core particles as clearly shown in the electron micrograph attached to the specificaiton of Japanese Patent Laid-Open SHO No. 62(1987)-226,162. The coating layer as such, therefore, does not completely cover the surface of the core particles (i.e. the coating layer lacks a compact texture). The toner consequently has a strong possibility of being prevented from acquiring a stable charging property by the influences of the coloring agent, magnetic powder, etc. contained in the core particles. Particularly when the toner has been stored or used under harsh temperature conditions, the components making up the core particles are suffered to pass through the gaps between the minute resin particles and finally exude from the surface of the toner to exert still more serious influences. Incidentally, this exudation of the components of the core particles to the toner surface concurrently entails agglomeration of toner particles as a problem.
Further, in the case of the pressure fixing capsulated toner, apart form the thermally fixing toner, improvements for surface properties have been proposed. Japanese Patent Laid-Open SHO No. 62(1987)-75,541, for example, discloses a pressure fixing capsulated toner which attains the improvement by forming a rugged surface on hard film shells enclosing pressure fixing cores. Generally, in the pressure fixing capsulated toner, the particles of this toner have a spherical form and a smooth surface, similarly to the thermally fixing toner obtained by the suspension polymerization described above and, therefore, entails such drawbacks as instability of charging property, pollution of the sensitive material, and poor cleaning property. The toner disclosed in the specification mentioned above, therefore, is aimed at overcoming these drawbacks by providing a rugged surface of the toner particles. As indicated in the specification, the rugged surface is formed by first depositing minute particles of silica, for example, on the surface of cores and subsequently forming a shell layer by the phase-separation method. By this procedure, however, the rugged surface cannot be formed easily because the degree of ruggedness tends to be influenced by the thickness of the shell layer and the minute particles are embedded completely by the shell layer if the shell layer has a large thickness. Conversely, if the shell layer has a small thickness, the minute particles deposited for the formation of a rugged surface tend to come off the surface and do not improve the surface properties sufficiently.
An object of this invention, therefore, is to provide a novel electrostatic latent image developing toner.
An another object of this invention is to provide an electrostatic latent image developing toner possessing not only stable charging property and high cleaning property but also high flowability.
A further object of this invention is to provide an electrostatic latent image developing toner which retains powder properties such as flowability, charging property, developing power, and cleaning property stably even when the particle diameter thereof is decreased enough to enhance the fineness of delineation for the sake of reproducibility of lines and improve the image quality in terms of granularity, mesh-pattern reproducibility, halftone reproducibility, tonality, and resolving power.